2 Answers
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The spots on the back of Ladybugs over the surface is defense mechanism to avoid predators. The spots come in different shapes and different numbers. Some say that those dots tell us their age. Since some ladybugs have 24 spots which means its age would be 24 years and that is not at all possible. So this is a popular misconception running around us.

But the real truth about the number of spots on the ladybugs' back is:

The number of spots on a ladybug does have significance. The spots and other markings do help you identify the species of ladybug. Some species have no spots at all. The record-holder for most spots is the 24-spot ladybug ( Subcoccinella 24-punctata), which has 24 spots, of course. Ladybugs aren't always red with black spots, either. The twice-stabbed ladybug ( Chilocorus stigma) is black with two red spots.

So now we know that the number of spots on the back of Ladybugs will help us to identify what species it belongs to. To give an insight into different species of them you can refer to this Identifying Ladybugs.

The above image shows different species of Ladybugs where you can see the varying number of spots in each bugs' back. More information can be found here Stripes on Ladybugs.

It is because of genes, and is completely inheritable! First I want point out that there spots, or points, do not change over time. Rather the spot number and location is fixed when Coccinellidae are pupa (tween aged insects).

You will notice I said Coccinellidae instead of ladybug. That is because "ladybugs" are actually several different species of insect. Thus we refer to the whole family of ladybugs, Coccinellidae. The spot number, shape, and color is different for different species, which gives you the variation in the number of points.